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Tuesday, June 23, 2015

Setting the Background of a WPF TextBox depending on Validation in a Template/Style using TemplateBinding

Recently I wanted to change the Background of a TextBox, if some own ValidationRule failed. But I couldn't get this to work. I tried several things and I gained more insights into styling and especially into styling wrong validated elements. And finally I get it to work.

If you are not familiar with WPF validation, here you can find excellent descriptions of how to use validation in WPF:

Validation.ErrorTemplate is used to add elements that decorate an existing element. So I cannot use it to change the Background of an existing element. But it can be used to add an image, an exclamation mark, TextBlock or something else next to the evaluated element.

Validation.HasError can be used to add a ToolTip or to change properties of an existing element.

So I should use it to change the color of the Background. But why is it not working?